168 H. SPENCER HARRISON. 



into the epidermis^ and are finally shed about the time of 

 hatching- (see later stages and fig. 12). In these features 

 Iguana appears to agree with Hatteria. 



The general resemblance to the functional teeth which is 

 exhibited by these embryonic teeth in their development 

 and structure, causes me to doubt their special homology 

 with the placoid scale, and I prefer to describe them as 

 simply belonging to the first or embryonic deutition, which is 

 no longer functional, but which, no doubt, was so when the 

 incubation period of Hatteria was shorter than it is at present. 



At Stage Q the teeth of the future functional series have 

 not yet begun their development. 



Stage R. 



Embryo R, 142, in about the fifth month of incubation. 

 Length of head from occipital condyle to premaxilla 6-5 mm. 



This embryo shows a very considerable advance on the last. 

 The teeth of the first series are now all calcified, and show 

 some degree of degeneration, inasmuch as many of them have 

 become solid nodules of dentine, lying immediately below the 

 epidermis, and having lost their pulp. The enamel epi- 

 thelium in some cases is still distinct, and an unmistakable 

 tendency to the formation of stellate tissue is occasionally to 

 be seen. The teeth number about fourteen above and fourteen 

 below, in addition to the palatine pair. We found in Stage 

 Q that there were only six of these teeth in a calcified con- 

 dition, the others being represented by a slightly greater 

 number of enamel organs than there are teeth at this stage. 

 In dealing with vestigial structures of this kind, little 

 morphological importance can probably be attached to such 

 differences in time of development, but it is interesting to 

 note that the first marginal teeth to calcify are those four which 

 lie near the anterior angles of the jaws, and which, in a 

 young animal, would be most useful in catching small prey. 



The teeth and enamel organs belonging to the future 

 functional series of the young animal are at this stage con- 



